EFFECTS OF MERCURY EXPOSURE ON CONDUCTANCE AND RESISTANCE VESSELS

Abstract

Exposure to mercury is a pro-inflammatory factor and causes oxidative stress. One of the main sites affected by oxidative stress, due to the development of atherosclerosis, is the aorta. In addition, changes in the reactivity of resistance arteries greatly influence blood pressure. This work aims to show the effects of mercury, in low concentrations, close to those found in environmental or occupationally exposed individuals, in conductance and resistance arteries. These low concentrations of mercury, for 30 days, caused increased vascular reactivity, oxidative stress and inflammation, as well as the reduction of NO bioavailability in both resistance and conductance arteries. Exposure to mercury has an important deleterious effect on vascular function, mainly because it induces endothelial dysfunction and vascular remodeling. This impact can be compared to those produced by traditional cardiovascular risk factors such as hypertension, diabetes and hypercholesterolemia.

Introduction

Mercury in organic, inorganic and elemental forms is used in various human activities and this causes man to be frequently exposed to this metal. Reports of the toxic effects of mercury are dating from the nineteenth century, however, only after the disasters in Japan (Minamata Bay) and Iraq in the 70s that more attention has been given to these effects.^1,2

The toxic effect of this metal is directly related to the type, time and route of exposure, and human exposure often occurs through the consumption of contaminated fish with organic mercury, administration of thimerosal in vaccines and inhalation of mercury vapor from dental amalgams.^3,4

Initially, it was observed that the renal and central nervous system had their functions affected by the exposure to mercury, however, in the last two decades also alterations in the cardiovascular system have been noticed, among which are highlighted arterial hypertension, carotid atherosclerosis, coronary artery disease and acute myocardial infarction.^5-9 The toxic effect of mercury on these systems is related to the higher generation of reactive oxygen species.^9-11

In experimental studies, our group has documented that direct infusion of mercury in rat caudal arteries produced a vasoconstriction associated with changes in endothelial function. These changes are evidenced by the reduction of the mediated endothelium-dependent relaxation and the increased production of COX-derived vasoconstricting prostanoids, increased free radical production with consequent reduction of NO bioavailability.¹² In this same arterial bed, nanomolar concentrations of mercury have already promoted increased reactivity to phenylephrine by increasing ACE activity and stimulation of Angiotensin II.¹³

A study conducted by Bastos et al. with the amazonian riverside population showed that almost all the habitants of the 40 cities studied have blood mercury concentrations above the reference values, indicating that human exposure to mercury is chronic and at low doses.¹⁴

Considering that humans are chronically exposed to mercury and little was known about the effect of this metal on the vascular system, our group developed an experimental model where the rats were exposed for 30 days to low doses of mercury mimecking human exposure to mercury and allowing the toxic effect of mercury on the vascular system to be evaluated.¹⁵

In the experimental model of chronic exposure to low doses of mercury developed by Wiggers et al,¹⁵ it was observed that at the end of treatment the animal had a blood concentration of mercury of approximately 8 ng/ml (29nmol/L), a concentration similar to that observed in the human population exposed to this metal.^4,16,17 No alteration of systolic blood pressure levels was observed in this model of exposure, however, there was an increase in the plasma activity of the angiotensin converting enzyme (ACE). With this experimental model it was possible to evaluate the vascular function of resistance and conductance arteries.

- Effects in resistance arteries

Peripheral vascular resistance is inversely proportional to the vessel radius to the fourth power and therefore decreases inthe size of these arteries can produce significant increases in peripheral resistance and therefore increased blood pressure.¹⁸ The luminous diameter is determined by the active and passive properties of the vessel. That is, by its morphological and functional alterations.¹⁹

Wiggers et al, have shown that treatment with low mercury concentration for 30 days induces endothelial dysfunction in resistance mesenteric arteries possibly by increasing oxidative stress, reducing NO bioavailability, and increasing superoxide anion production derived from NADPH oxidase. Among the main results of the functional evaluation, it is highlighted that the chronic exposure to mercury caused: 1) increased vasoconstricting response to phenylephrine and reduced endothelial NO modulation of this response; 2) reduction of ACh-induced endothelium-dependent vasodilatory response; 3) increased superoxide anion production, plasma malondialdehyde and total antioxidant status; 4) restoration of NO endothelial modulation in the contractile response to phenylephrine and ACh-induced vasodilation in the presence of SOD (superoxide anion scavenger) and apocynin (NADPH oxidase inhibitor).¹⁵

It is known that cardiovascular changes, such as hypertension, in addition to having oxidative stress and endothelial dysfunction, are accompanied by structural changes in resistance vessels, a process known as vascular remodeling, usually associated to decreases in its diameter.^20-23 Vascular remodeling is a complex process that may involve an increase (hypertrophy), decrease (hypotrophy) or rearrangement (eutrophy) of vascular wall material.^21,24 In the literature, there was no experimental evidence that chronic treatment with HgCl2 could be associated with structural or mechanical changes of resistance vessels. Our study evaluated for the first time the structural and mechanical properties of third order mesenteric arteries of Control rats and treated with HgCl2 in a system for pressurized arteries, which represents one of the most appropriate current methods for this type of study for its approach to in vivo conditions.²⁵

The mercury treatment produced a reduction in wall thickness and in the media: lumen ratio of the vessels, as well as an increase in internal diameter, which could be a consequence of a decrease in vascular wall thickness.¹⁵ These parameters correspond to the definition of outward hypotrophic remodeling.²⁶

– Conductance arteries

Exposure to mercury, as demonstrated in resistance arteries and other biological systems, is known to be an important cause of oxidative stress and inflammation.^9,11,15,27 Oxidative stress and inflammation are important factors that generate LDL cholesterol which, in conductance arteries, will lead to atherosclerosis.^28,29 Thus, the effects on conductance arteries, such as the aorta, together with the changes observed in resistance arteries are important for the study of metal exposure, since endothelial dysfunction in both are added as factors of cardiovascular risk.

This same experimental model was used to evaluate the effect of mercury on the conductance arteries and, for this, experiments of vascular reactivity were developed in segments of the thoracic aorta. In this vascular bed, it was also observed that exposure to low doses of mercury during 30 days caused an increase in the contractile response to phenylephrine and a lower participation of NO in this response, reduction of the vasodilator response to acetylcholine, increase in superoxide anion production and malondialdehyde levels (MDA). In addition, it was observed that in the presence of apocynin (NADPH oxidase inhibitor) and SOD (superoxide anion scavenger), there was an improvement of the endothelial NO modulation in the contractile response to phenylephrine and an increase in the vasodilator response to acetylcholine. These results suggest that exposure to mercury promoted endothelial dysfunction due to the reduction of NO bioavailability due to the increase in the production of reactive oxygen species by NADPH oxidase.¹⁵

The participation of the angiotensin II pathway in the vascular alterations caused by mercury had already been observed by our group.¹³ Subsequently, other authors have demonstrated that angiotensin II was able to induce COX-2 expression, to increase prostanoid production^30,31 and that contractile prostanoids derived from the COX-2 pathway caused endothelial dysfunction.^32-34 To elucidate the role of angiotensin II and contractile prostanoids derived from the COX-2 pathway in the vascular reactivity of rats exposed to mercury Peçanha et al, conducted experiments on aortic artery segments.

The hypothesis that exposure to mercury promotes a greater participation of the angiotensin II pathway in vascular reactivity was was reinforced because in rats exposed to mercury it was observed: a) increased plasma ACE activity; B) reduction of the contractile response to phenylephrine in the presence of Losartan (AT 1 receptor antagonist). In addition, the presence of indomethacin (non-selective COX inhibitor), NS 398 (COX-2 inhibitor), SQ 29,548 (TP receptor antagonist), furegrelate (TXA2 synthase inhibitor), SC 19,220 (EP 1 receptor antagonist) reduced the contractile response to phenylephrine in aortic segments of rats exposed to mercury and promoted increased gene expression of COX-2 in the aortas of these animals. These results indicate the involvement of the angiotensin II pathway and the COX-2 pathway, especially TXA2 and PGE2, in the vascular alterations (vascular reactivity) caused by mercury contamination.³⁵

Later, in a study where chronically exposed rats at low doses of mercury were co-treated with apocynin (nonspecific NADPH oxidase inhibitor), it was demonstrated for the first time that apocynin improved the endothelial function of the aorta from rats chronically exposed to mercury.³¹ This effect is due to the prevention of increased oxidative stress, reduction of lipid peroxidation, changes in defense mechanisms and consequent improvement of NO bioavailability. It is noteworthy that treatment with apocynin did not reduce the participation of COX-2-derived prostanoids in the contractile response to phenylephrine. These results reinforce the hypothesis that chronic exposure to mercury promotes a greater participation of these prostanoids and reactive oxygen species in the contractile response of the rat aorta to phenylephrine. In addition, they indicate that mercury acts via two different pathways, COX-2 and NADPH oxidase pathways, independently.³⁶

A more recent study from our group showed that the effects of mercury exposure on vascular reactivity is concentration dependent. Since exposure to a concentration 2.5 times greater than the experimental model proposed by Wiggers et al,¹⁵ causes an increase in vascular reactivity of the aorta, but half of the concentration promotes a reduction in the contractile response to phenylephrine. This reduction is probably due to an increase in NO bioavailability as reinforced by the protein expression data of the phosphorylated fraction of eNOS in aortic segments.³⁷

However, when mercury (6nM) was added directly to the perfusion medium of aortic arteries, it was observed that there was still an increase in reactivity to phenylephrine due to increased oxidative stress and consequent reduction of NO bioavailability.³⁸ This concentration is much lower than the plasma concentration achieved by the treatment proposed by our group previously (29nM or 8ng/mL).¹⁵

That is, the effects shown by Azevedo et al,³⁷ were surprising, suggesting that chronic and controlled exposure of mercury at concentrations that do not lead to endothelial dysfunction promotes adaptive and protective responses of the arterial vasculature.³⁷

These results show the deleterious effects of mercury on the vascular system, even at low concentrations, help to clarify the mechanisms by which this metal exerts its effects and already suggest possible therapeutic pathways.

Exposure to low concentrations of mercury has a negative effect on vascular function, mainly because it induces endothelial dysfunction and vascular remodeling. This impact can be compared to those produced by traditional cardiovascular risk factors such as hypertension, diabetes and hypercholesterolemia. Therefore, mercury can be considered an important risk factor for cardiovascular disease that can participate in the development of cardio and cerebrovascular events. Whether these effects increase the consequences of traditional risk factors or play a primary role in patients with low cardiovascular risk still needs to be better studied.

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Authors

Lorena Barros Furieri¹; Franck Maciel Peçanha²; Giulia Alessandra Wiggers³

¹PhD, Professor of Departament of Nursing, Federal University of Espírito Santo, Vitória, ES, Brazil.

^2,3Doctor, Professor of Federal University of Pampa, Uruguaiana, RS, Brazil.